Enigma Team is an alternative game mode for FIST that focuses on intelligence gathering and espionage over action-first tactical engagements.
The PDF is 16 pages, with a straightforward layout and dense but well organized text that's easy to read.
Contents-wise, Enigma Team adds a new wrinkle to the lore, where FIST isn't just an organization composed of freewheeling do-gooder mercenary strike teams. It also works in conjunction with Enigma Teams, who gather information for those mercenaries to use.
The main mechanical feature of Enigma Team is investigative trainings, which give a bonus to certain avenues of investigation. There's a long list to pull from, with some headings spelled out in more detail than others.
There's also a detailed list of other sources teams can call on for help, ranging from computers to aliens to summoned spirits.
For GMs, there's a lot of discussion on how to structure an investigation, and how to make its moving parts satisfying for the players to interact with. None of this is really mechanical, but it's good advice, and worth the read.
Overall, if you want to play a more gumeshoe-y version of FIST, or if you want some material for an unusual variant mission to break your normal campaign's structure, I'd recommend giving Enigma Team a shot.